1. Field of the Invention
The present invention relates to a fuel injector for an internal-combustion engine.
In particular, the present invention relates to an injector of the type comprising a hollow injector body, which has an axis of its own and delimits a seat for housing an injection-control valve. The valve in turn comprises a valve body of a tubular shape inserted and blocked inside the seat of the injector body coaxially to the cited axis by means of a ring-nut screwed into the seat on the injector body to force an external flange of the valve body against an internal shoulder of the injector body itself. The valve further comprises an open/close element pressed against a head surface of the valve body by an elastic thrust element, and an electric actuator set along the aforesaid axis to exert an action countering the one exerted by the elastic element and to enable the open/close element to set itself at a distance from the aforesaid head surface. Between the valve body and the injector body there is obtained an annular chamber, into which there is an outlet for pipe for supply of fuel under pressure to be injected. The chamber, which shares the aforesaid axis, is delimited in an axial direction by two shoulders facing one another, one of which is carried by the injector body and the other by the valve body. For the purpose of preventing leakage of fuel between the injector body and the valve body, against the shoulder of the injector body there is set a ring seal made of elastomeric material. On the shoulder of the valve body there acts, instead, in use, the pressure of the fuel contained in the annular chamber, generating an axial thrust directed towards the ring-nut.
2. Description of the Related Art
Albeit universally used, known injectors of the type described above are far from efficient and reliable and hence are not altogether satisfactory, above all when the regulation of the injection is carried out with particularly contained strokes or lifts of the open/close element.
The inefficiency can basically put down to the fact that, on account of the particular conformation of the annular seat that receives the fuel under pressure, the fuel itself generates on the valve body axial thrusts opposite to the elastic thrust exerted by the elastic body on the open/close element. Said thrusts, which are proportional to the pressure of introduction of the fuel, are unloaded on the injector body through the ring-nut and deform the valve body elastically. Consequently, during operation of the injector, the open/close element and the electric actuator are set in relative positions different from the ones envisaged in the design stage, and hence the effective lifts of the open/close element, i.e., in operating conditions, are different from the ones envisaged.
Furthermore, since the elastic deformations of the valve body are normally of the order of a few micron and strictly linked to the instantaneous value of the pressure of the fuel in the annular chamber, they do not represent a fixed offset with respect to the static situation in the absence of pressure, and hence can in no way be compensated for in the stage of setting of the injector.